This invention relates to a light alloy piston for internal combustion engines, which piston has a cambered oval external shape and has load-carrying skirt surfaces each of which is divided into two load-carrying partial skirt surfaces by a peripherally extending depression.
At the present time, almost all light alloy pistons for internal combustion engines have a sliding surface which has been machined to a predetermined shape in consideration of the thermal and mechanical conditions arising during the operation of the engine. For this reason pistons of all sizes and types have been provided with a large number of cambered oval external shapes with the object to minimize the running clearance while retaining a high seizure-proofness. The cambered shape of the piston in the cold is due to a curved generatrix of the skirt of the piston. Such a curved generatrix is required for a compensation of the temperature pattern along the piston skirt by the resulting expansion. Owing to the relatively high temperature resulting at the top end of the piston skirt during the operation of the engine and in order to ensure good lubricating conditions, i.e., the formation of a lubricant wedge, that curved generatrix is strongly tapered at said top end whereas it is slightly curved in the middle. In consideration of the lubricating conditions that curved generatrix may also be slightly tapered or may be straight at the bottom end of the skirt.
Owing to the ovalness of the piston skirt having a minor axis in the direction of the axis of the piston pin, the temperature-dependent increases of the volume and girth of the piston are deflected in the axial direction of the piston pin and deformations of the skirt which are due to the force exerted on the sliding surface will be taken up. The piston skirt must have an ovalness reserve even when it is fully stressed during the operation of the engine. Because the connecting rod performs a pivotal movement, which results at the sliding surface in the exertion of a variable force that is due to the resolution of the gas force and acts in a direction that is at right angles to the axis of the piston pin, the piston carries load during the operation of the engine mainly in the regions which are on both sides of the pressure-backpressure plane which includes the axis of the piston pin and extends at right angles thereto in the so-called pressure-backpressure direction. In said regions the formation of a hydrodynamic lubricant film is not ensured throughout the stroke of the piston so that mixed friction between the piston and the cylinder surface occurs near the top and bottom dead centers. As a result, a wear mark which is generally described as a wear pattern having approximately parabolic lateral boundary lines is formed on the pressure and backpressure sides of the piston skirt. Said boundary lines extend on each side of the pressure-backpressure plane over up to 45.degree. in each half of the piston skirt. In a piston designed in consideration of the above aspects, the piston skirt is caused to contact the cylinder surface over an area which is as large as possible in order to ensure that the lateral forces will be taken up. The mixed friction which is apparent from the wear pattern of the piston skirt and the hydrodynamic friction which occurs between the piston and the cylinder wall and is due to shearing forces exerted on the lubricant film by the relatively sliding members which are not in direct contact with each other result in friction losses, which may amount to as much as 15% of the total power of the internal combustion engine. Whereas the forces which are due to hydrodynamic friction are generally smaller than the forces which are due to mixed friction, the former account for a major share of the friction loss because the velocity of the sliding piston results in an occurrence of hydrodynamic lubrication during a major part of the stroke. As the hydrodynamic friction is approximately proportional to the surface area of the load-carrying lubricant film, a decrease of the load-carrying skirt area may result in an increase of the power of the internal combustion engine and/or in a saving of fuel.
For instance, it has been proposed in the periodical Kraftfahrzeugtechnik, No. 12, 1976, Berlin, on pages 364 to 367, to reduce the friction at the piston skirt in that each load-carrying skirt surface is divided into two load-carrying surfaces by the provision of respective slide rings near the top and bottom ends of the skirt. Published German Application No. 32 28 982 discloses a piston in which the piston is provided on both sides of the pressure-backpressure plane with one or with a plurality of separate, relatively small bearing surfaces, which are in contact with the cylinder surface. The edge portions of the bearing surfaces are so designed that the lubricant will be conducted to the associated bearing surface and between the piston and the cylinder surface. Just as in the piston disclosed in the periodical Kraftfahrzeugtechnik, No. 12, 1976, that design results under full load in a higher pressure per unit of area so that the wear will be increased and seizure may occur because a contact can be established only at discrete bearing surfaces and reserve bearing surfaces are not provided. Even when the bearing surfaces are so dimensioned that a satisfactory function is ensured under normal operating conditions, there may be a seizure and premature wear at the bearing surfaces under an overload or under undesirable lubricating conditions as may sometimes arise in practice.
MTZ Motortechnische Zeitschrift 40 (1979), 10 discloses on page 462 a production piston in which the load-carrying skirt surface has been reduced by about 59% by the provision of a ground clearance surface in the direction of the pin axis and by an annular recess. Whereas this results in a reduction of the fuel consumption by about 6% at 50 km/h, it involves an undesired higher consumption of lubricant and adversely affects the guidance of the piston.